The present invention relates to an active matrix substrate and a method for fabricating it, and more particularly to an active matrix substrate suited for a display device and a method for fabricating it. The active matrix substrate is a substrate which includes switching elements on the crossings of the matrix.
Electronic apparatus having a thin film semiconductor device such as an active matrix substrate, in which thin film transistors (TFT's) or the like are formed on an insulating substrate (e.g. glass), have important technical problems such as improvement of image quality (improvement of aperture of pixels) and ease of fabrication, with the development of high definition of images. In order to solve these technical problems, there have been proposed several contrivances. Examples of the proposals relative to the active matrix substrate are disclosed in JP-A-61-229365, JP-A-61-231765, JP-A-61-278163, and JUM-A-62-120354.
Rough explanation will be given for the method for fabricating an active matrix disclosed in these references and problems relative thereto. First, after a Poly-Si film, which serves as an active layer of the TFT having a MOSFET structure, has been formed, an SiO.sub.2 film constituting a gate insulating film and a Poly-Si film constituting a gate electrode are successively deposited thereon. The Poly-Si film for a gate electrode and the SiO.sub.2 film for a gate insulating film will be subjected to photoetching. More specifically, they are patterned using photoresist as a mask. After the photoresist has been removed, platinum (Pt) is deposited on the entire surface of the structure. The structure is then heat-treated to form a Pt silicide layer. The unreacted platinum is removed using aqua regia.
In this case, the following problems will be involved. The Pt silicide layer will be formed on the surface of the Poly-Si layer for a gate electrode to thereby realize low resistance of the Poly-Si layer, but the surface of the Poly-Si layer serving as an active layer (including source and drain regions of MOSFET as well as a channel region thereof) will be silicided. Thus, source and drain electrodes connected with the source and drain regions will provide Schottky junctions, which makes it difficult to obtain a TFT with an improved characteristic. As a result, a redundant step is required to remove the silicide layer from the source and drain regions, and it is difficult to completely remove this silicide layer. Moreover, if the Poly-Si film for an active layer is made thin in order to prevent the characteristic of the TFT from being greatly varied during irradiation of light, the Poly-Si film will exfoliate due to the presence of the silicide formed thereon.